The present invention further relates to a controller for controlling a temperature downstream of a catalyst in the exhaust tract of an internal combustion engine including a first, outer control loop in which the controller calculates a first control variable from a first deviation that is calculated by the controller from a first actual value and a first setpoint value; a measure of a temperature downstream of the catalyst being used as the first actual value.
Such a method and controller are described in the publication “Fortschritt-Berichte VDI, Reihe 12 Verkehrstechnik/Fahrzeugtechnik, Nr. 49, 23. Internationales Wiener Motorensymposium, 25.–26. Apr. 2002, Seite 171 [VDI Progress Reports, series 12, Traffic Engineering/Vehicle Engineering, issue 49, 23, International Vienna Motor Symposium, Apr. 25–26, 2002, page 171]”; however, no details of the control are disclosed there.
Modern emission control systems generally feature a plurality of catalysts and/or filters arranged one behind the other. Thus, for example, NOx storage catalysts and particulate filters are arranged downstream of a three-way catalyst, an oxidation catalyst, or a primary catalyst in the direction of exhaust gas flow. In order for the rear catalysts in the direction of flow to function properly, specific exhaust gas temperatures are, at least temporarily, required at the inlet to these catalysts.
Thus, for example, a NOx storage catalyst, which stores nitrogen oxides when the exhaust gas is lean, is regenerated by periodically producing oxygen deficiency in the exhaust gas. Increased exhaust gas temperature promotes the regeneration. Particulate filters, such as are increasingly used in motor vehicles with diesel engines, are another example of emission control components that require certain minimum temperatures to remain functional.
To be able to maintain the absorption capacity of a particulate filter for soot over longer periods of time, the soot stored in the particulate filter must, from time to time, be burned to CO2 at an elevated exhaust gas temperature. To this end, the particulate filter must, at least occasionally, be heated to above 550° C. Frequently, the particulate filter is connected to an upstream oxidation catalyst. A temperature sensor located between the oxidation catalyst and the particulate filter does provide a very accurate value for the temperature at the inlet of the particulate filter, but, due to the large heat capacity of the upstream oxidation catalyst, the temperature sensor responds only very slowly to changes in the exhaust gas temperature that are controlled upstream of the oxidation catalyst. This makes control of the exhaust gas temperature at the inlet of the particulate filter so sluggish that the response of the control to changes in the exhaust gas temperature is only fast enough when the internal combustion engine is in steady-state operation. Since internal combustion engines in motor vehicles generally operate with rapidly varying loads and speeds involving rapid changes in the exhaust gas temperature, steady-state conditions are more of an exception than a rule. Because of this, proper regeneration of the particulate filter during normal operation of the motor vehicle becomes more difficult.
Against this background, it is an object of the present invention to provide a method and controller for exhaust gas temperature control, allowing improved control accuracy even during transient operating conditions involving exhaust gas temperatures that vary strongly when not actively controlled.
In a method of the type mentioned at the outset, this objective is achieved by a second, inner control loop in which at least one second control variable is calculated from a second deviation that is calculated from a second actual value and a second setpoint value; a temperature upstream of the catalyst being determined as the second actual value, and the second control variable influencing an intra-engine heat generation.
Moreover, in a controller of the type mentioned at the outset, this object is achieved in that the controller, in a second, inner control loop, calculates a second control variable from a second deviation that is calculated by the controller from a second actual value and a second setpoint value; a temperature upstream of the catalyst being used as the second actual value, and the second control variable influencing an intra-engine heat generation.